Waterproof microphone



u y 29, 1952 R. M. GOGOLICK EI'AL 2,605,346

WATERPROOF MICROPHONE Filed Sept. 18, 1945 FIG.

ATT RNEY Patented July 29, 1952 WATERPROOF MICROPHONE Roland M. Gogolick, Cambridge, and Richard L. Brown, Arlington, Mass, assignors to the United States of America as represented by the Score tary of the Navy Application September 18, 1945, Serial No. 617,153

This invention relates to microphones and improvements in methods of their manufacture and is illustrated herein as embodied in a compact microphone having a coating of insulation bonded directly to its transducer.

There has been a need for a satisfactory hydrophone for field measurements of underwater sound. The usual method of waterproofing a hydrophone impairs its inherent sensitivity and tends to make it directional because of the bulk of the waterproofing and the damping eifect of air spaces between the water and the transducer. On the other hand, those hydrophones which are sensitive, are relatively fragile and diflicult to construct.

Therefore, itis an object of this invention to provide an inexpensive, easily constructed, and rugged hydrophone which is. relatively nondirectional and in which the connection between a microphone of this type is made by applying layers of insulation thereto, between which layers is disposed an electrostatic shield. More specifically after one layer of insulation has been applied, a layer of conducting material, constituting the shield, is painted over the insulation. Then a further layer of insulation isapplied by a second dippingm Thus, a coating is produced which is firmly bonded to the microphone, and the coating acts as a diaphragm attached directly to the transducer with no air spaces between the transducer and the waterproof coating. V

The invention is described with reference to the use of a transducer including Rochelle salt piezo-electric crystals, but other types of transducers may be utilized without departing from the spirit of this invention.

With the above and other objects and features in view, the invention will now be described with reference to the accompanying drawings which illustrate ,a preferred embodiment of the invention and will be pointed out in the claims.

7 Claims. (01; 177-386) thecondition shown in Fig. l.

Fig. 1 is a perspective view of a microphone constructed in accordance with the invention in lcondition to receive a coating of insulation.

Fig. 2 is a view of a finished microphone shown partly in cross section to illustrate the layers of constructed of a Bakelite'type resin or other suitable solid non-conductor, which, when assembled, forms a substantially rectangular block. The

two halves of the holder are held together by a vbolt l.

A crystal bank 9 is held in a transverse opening ll between the two halves of the holder, the opening being of generally rectangular shape and extending completely across the holder as shown. As illustrated, a 4-ply Rochelle salt crystal bank is used as a piezo-electric transducer, the crystal bank being of such a length as to extend the entire length of the transverse, opening whereby two faces of the crystal bank are flush with two opposite faces of the holder, as more clearly shown in Fig. 2.

Two leads l3 and [5 are connected with the crystal bank 9 and pass through openings l1 and I9, between the halves of the holder to connect the crystal bank with an electrical cable 2|.

The cable 2| is a shielded electrical cable of the usual underwater type having a pair of insulated leads 23 and 25 surrounded by a metallic sheath 2'! which in turn is covered by an outer waterproof cover 29. The cable 21 connects the microphone with appropriate amplifiers and recording or listening devices (not shown).

The microphone is given a coating of waterproof insulation by dipping it in a neoprene paint. The cable is prepared for dipping by baring the tip portions 3| of the leads and a portion 33 of the shield 21 to put the microphone and cable in The microphone is cleaned with benzol, carbon tetrachloride, or an equivalent solvent. Then, the first coat of neoprene paint is applied. The microphone is immersed in a non-aqueous solution of a neoprene paint a distance sufiicient to cover the ex- "tective and insulating first layer 35 (Fig. 2). A

non-aqueous and substantially anhydrous solution is used because small amounts of water affect a Rochelle salt piezo-electric transducer ad- "versely.

previously applied.

When this insulating layer 35 has dried, a

shielding layer 3'! is applied. The microphone is integral with the sheath 2'! but insulated from the crystal bank 9 by the neoprene layer 35 When the shielding layer 3-! has dried, the

microphone is again dipped in neoprene paint. i I

For this second dip, the microphone is immersed to a sufficient depth to cover all the microphone which was covered with the conducting paint and also to cover part of the waterproof cover 29 of the cable 2| to give an outer waterproof covering 39 completely surrounding the microphone which is integral with the outer cover of the cable. Thecovering is allowed to dry and cure for several days. If required, two or more layers may be applied. When the outer covering has cured sufficiently, it is dusted with powdered talc to eliminate tackiness and is ready for use.

The completed microphone is shown in Fig. 2. Surrounding the holder I and the crystal bank 9 are three layers, the layer'of insulation 35, the electrical shielding layer 3'! and the outer waterproof covering 38 all bonded tightly together and to the unrestrained faces of the crystal bank 8. There is a continuous solid connection between the crystal bank 9 and the outer protective waterproof covering 39 unbroken by air spaces, and the layers act as a diaphragm conducting sound vibrations to the crystal bank with little loss in intensity.

This microphone has excellent electrical re- 'sponse. Experiments with microphones conbeen tested under fluid pressures up to 600 lbs.

per square inch. In conventionalv types failure is caused at such pressures by collapse'of the diaphragm or breaking of the fluid seal. In this instrument, due to the intimate bonding of the neoprene coating to the transducer, there is a fluid seal provided without the use of a mechanical diaphragm and the microphone is therefore more rugged than those of other types.

This microphone has been found to be of par-'- ticular value in underwater sound reception and measurement, but may be used satisfactorily for other purposes as well.

Having described our invention what we claim as novel and desire to protect by Letters Patent of the United States is:

l. A microphone comprising a transducer, a holder for said transducer, a cable connected to said transducer, said cable having an electrostatic shielding sheath and an outer waterproof cover, a layer of thin, flexible, waterproof insulation surrounding said transducer and said holder, a layer of thin, flexible, electrostatic shielding material surrounding said insulation and engaging said sheath, and a second layer of thin, flexible, insulation surrounding said shielding material and engaging said outer waterproof cover, said layers of insulation being bonded to said layer of shielding material, said first-mentioned layer of insulation being bonded to said transducer, the composite coating of said layers of insulation and shielding material thereby being adapted to vibrate integrally with said transducer.

2. A hydrophone comprising a piezoelectric crystal, holding means for said crystal comprising a substantially rectangular block of insulating material having an opening therethrough conforming to the size and shape of said crystal, said holding means being arranged to exelectrostatic shield imbedded in said layer to.

completely surround said crystal and, said holding means and engaging the electrostatic shield of said cable. 1

3. A hydrophone comprising, in combination, a piezoelectric transducer, a holder of insulating material firmly contacting said transducer to expose two faces thereof, a two conductor cable having its two conductors individually insulated, ametal sheath enclosing both said conductors and an outer insulating sheath surrounding said metal sheath, said two conductors having the insulation removed froma short length on an end thereof and extending into said holder for electrical connectionwith said transducer, and said metal'sheath and said outer sheath being removed from successively slightly longer lengths, respectively, from said end of said cable, a first, thin, flexible layer of waterproof insulating material completely covering said holder and transducer and the exposed insulation of the individual conductors of said cable and being adhesively bonded thereto, a thin, flexible layer of conducting material completely surrounding said first layer and circumferentially contacting said metal sheath of said cable, and a second, thin, flexible layer of waterproof insulating material completely surrounding said. layer of conducting material and the end region of the outer insulating sheath of said cable, said first and second layers of insulation and said conducting layer being adhesively bonded to each other whereby the composite coating is adapted to vibrate integrally with said transducer.

4. A hydrophone comprising, a piezoelectric crystal, means for-holding said crystal to expose two vibratory faces thereof, a cable having an electrostatic shielding sheath and an outer waterproof cover connected to said crystal, 9.

.first layer of waterproof insulation surrounding said crystal and said holding means and bonded to the exposed faces of said crystal, a thin flexible layer of shielding material surrounding said first layer of insulation and engaging the sheath of said cable, and a second layer of waterproof insulation surrounding saidshielding material and bonded thereto and engaging the outer cover of said cable, the composite coating of said layers forming a diaphragm adapted to vibrate integrally with said crystal.

5. A microphone comprising, a piezoelectric crystal, a holder for said crystal, a cable having an electrostatic shielding sheath and an outer waterproof cover connected to said crystal, a first thin layer of flexible waterproof insulation surrounding said crystal and said holder, a thin layer of flexible electrostatic shielding material surrounding said first layer of insulation and engaging said sheath, and a second thin layer of flexible insulation surrounding said shielding material and engaging the outer cover of said cable, said first layer of insulation being bonded to said crystal and holder and to said electrostatic shielding material, said second layer also being bonded to said electrostatic shielding material, the composite coating of said layers of insulation and shielding material thereby being adapted to vibrate integrally with said crystal.

6. A hydrophone comprising a bank of piezoelectric crystals, a holder for said crystals comprising a split-block of insulating material of generally rectangular shape having an opening therethrough shaped to firmly hold said bank of crystals to expose two vibratory faces thereof, a two conductor cable having an electrostatic shield and a, waterproof outer cover, the conductors of said cable being positioned between two crystals of said bank and maintained in firm electrical contact therewith by said holder, a first thin layer of flexible waterproof insulation completely covering said crystals and said holder and being bonded thereto, a thin, flexible layer of conducting material surrounding said first layer and engaging the electrostatic shield of said cable, and a second thin layer of flexible waterproof insulation surrounding said layer of conducting material and engaging the outer cover of said cable, said layers being adhesively bonded to each other whereby the composite coating is adapted to vibrate integrally with the vibratory faces of said crystals.

7. A hydrophone comprising a piezoelectric crystal, a holder of insulating material firmly contacting said crystal to expose two vibratory faces thereof, a two conductor cable having an electrostatic shielding sheath and an outer waterproof cover, said conductors being bared of their insulation at one end and firmly held in contact with said crystal by said holder, a first thin layer of neoprene completely covering said crystal and said holder and engaging the insulation on said two conductors, a thin flexible layer of metal completely surrounding said first layer of neoprene and engaging the electrostatic shield of said cable, and a second thin layer of neoprene completely surrounding said layer of metal and engaging the outer cover of said cable, said layers being bonded together whereby the composite coating is adapted to vibrate integrally with the vibratory faces of said crystal.

ROLAND M. GOGOLICK. RICHARD L. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,002,903 Davison Sept. 12, 1911 1,658,327 Dodge Feb. 7, 1928 2,138,036 Kunze Nov. 29, 1938 2,222,729 Ver Planck et a1. Nov. 26, 1940 2,402,531 Christian June 25, 1946 2,413,462 Massa Dec. 31, 1946 2,420,311 Gowell May 31, 1947 2,427,062 Massa Sept. 9, 1947 2,427,348 Bond et al Sept. 16, 1947 2,433,383 Mason et a1. Dec. 30, 1947 

